Handover (HO) means that a mobile station moves from a wireless interface of a base station to a wireless interface of another base station. Hereinafter, a handover procedure in a general Institute of Electrical and Electronics Engineers (IEEE) 802.16 system will be described.
In an IEEE 802.16 network, a serving base station (SBS) may broadcast neighbor base station information through a neighbor advertisement (MOB_NBR-ADV) message to inform a mobile station (MS) of information (topology) about a fundamental network configuration.
The MOB_NBR-ADV message contains system information, such as a preamble index, frequency, a degree of handover (HO) optimization, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, regarding the serving base station and neighbor base stations.
The DCD/UCD information includes information that the mobile station should know in order to transmit and receive information through a downlink and an uplink. For example, the DCD/UCD information includes handover (HO) trigger information, a Medium Access Control (MAC) version of a base station, Media Independent Handover (MIH) capability, etc.
A general MOB_NBR-ADV message contains only information regarding IEEE 802.16e type neighbor base stations. As a result, information regarding other type neighbor base stations excluding the IEEE 802.16e type neighbor base stations may be broadcast to mobile stations through a Service Identity Information ADVertisement (SII-ADV) message. Consequently, the mobile station may request the serving base station to transmit the SII-ADV message to acquire information regarding this type network base station.
A process of a mobile station which acquires information of neighbor base stations through the aforementioned method performing handover in an IEEE 802.16 network will hereinafter be described in more detail with reference to FIG. 1.
FIG. 1 is a view showing an example of a handover procedure which may be performed in a general IEEE 802.16 system.
Referring to FIG. 1, a mobile station (MS) may be accessed to a serving base station (SBS) to exchange data with the serving base station (S101).
The serving base station may periodically broadcast information regarding a neighbor base station in which the serving base station is located to the mobile station through a MOB_NBR-ADV message (S102).
The mobile station may start to scan candidate handover base stations (HO BSs) using a handover (HO) trigger condition while communicating with the serving base station. The mobile station may transmit a handover request (MOB_MSHO-REQ) message to request the serving base station to perform a handover procedure when a handover condition is satisfied, for example, when a predetermined hysteresis margin value is exceeded (S103).
The serving base station may inform the candidate handover base stations (HO BSs) contained in the MOB_MSHO-REQ message of handover request from the mobile station through an HO-REQ message (S104).
The candidate handover base stations (HO BSs) may perform pre-processing for the mobile station that requests the handover and transmit information regarding the handover to the serving base station through an HO-RSP message (S105).
The serving base station may transmit the information regarding the handover, acquired through the HO-RSP message, from the candidate handover base stations to the mobile station through a handover response (MOB_BSHO-RSP) message. The MOB_BSHO-RSP message may contain information required to perform the handover, such as action time, a handover identifier (HO-ID), and a dedicated handover code division multiple access (CDMA) ranging code (Dedicated HO CDMA ranging code) (S106).
The mobile station may determine one target base station from among the candidate handover base stations based on the information contained in the MOB_BSHO-RSP message received from the serving base station. As a result, the mobile station may try to perform ranging by transmitting a CDMA code to the determined target base station (S107).
Upon reception of the CDMA code, the target base station may transmit information representing whether or not ranging has been successfully performed and physical correction values to the mobile station through a ranging response (RNG-RSP) message (S108).
Subsequently, the mobile station may transmit a ranging request (RNG-REQ) message for authentication to the target base station (S109).
Upon reception of the ranging request message from the mobile station, the target base station may provide system information, such as a Connection Identifier (CID), which may be used in the corresponding base station to the mobile station through the ranging response message (S110).
When the target base station has successfully performed authentication of the mobile station and transmitted all update information, the target base station may inform the serving base station of the mobile station whether or not the handover has been successfully performed through a handover completion (HO-CMPT) message (S111).
Subsequently, the mobile station may exchange information with the target base station to which the handover has been performed (S112).
It is assumed that the above-described handover process is performed between a mobile station and a base station conforming to an IEEE 802.16e system (WirelessMAN-OFDMA R1 Reference System). In a handover procedure defined in an IEEE 802.16m (WirelessMAN-Advanced Air Interface) system, kinds of some medium access control (MAC) management messages and parameters contained in the MAC management messages may be different from each other. For example, ranging request/response (RNG-REQ/RSP) messages may be replaced with advanced ranging request/response (AAI-RNG-REQ/RSP) messages and a handover response (BSHO-RSP) message may be replaced with a handover command (AAI-HO-CMD) message.
When a mobile station initially and directly transmits an uplink signal to a target base station (for example, when the mobile station transmits the ranging code as in step S107) during an handover process, the uplink signal is transmitted using default power predefined by a system since the optimum transmit power is not recognized. If the power is too low, the signal may not be correctly transmitted. On the other hand, if the power is too high, the signal may interfere with a signal transmitted by another mobile station or a battery of the mobile station may be consumed. For this reason, optimum uplink power control is very important. In the IEEE 802.16m system, the base station transmits a ranging acknowledgement (AAI_RNG-ACK) message to the mobile station as a response to the ranging code transmitted from the mobile station. The mobile station may determine whether or not the ranging code has been detected and recognize information, such as physical correction values, and determine whether it is necessary to retransmit the ranging code through the ranging acknowledgement message. In a case in which the default power is not suitable for a channel environment between the current mobile station and the base station, a ranging status field of the ranging acknowledgement message is set to “continue” and a power correction value is contained in the corresponding message. In this case, the mobile station retransmits the ranging code to the target base station using transmit power, to which the correction value has been applied. During this process, however, unnecessary delay time is generated. For this reason, an effective uplink power determination method to solve the above problem is required. The problem related to power control may be similarly generated even in a case in which the mobile station performs zone switch in a base station operating in a mixed mode.